1. Field of the Invention
The present invention relates to an apparatus and method for detecting ionic materials, and more particularly, to an apparatus having reduced noise and a method of using the apparatus for detecting ionic materials.
2. Description of the Related Art
Transistor-based biosensors use electric signals to detect ionic materials such as biomolecules. Transistor-based biosensors are manufactured using a semiconductor formation process, and have an advantage of rapid conversion of sensed signals into electrical signals. Thus, a considerable amount of research has been conducted into manufacturing transistor-based biosensors.
A biosensor for detecting biological reactions using a field effect transistor (“FET”) is disclosed in U.S. Pat. No. 4,238,757. According to the biosensor for detecting biological reactions of the prior art in U.S. Pat. No. 4,238,757, a surface charge concentration changes due to an antigen-antibody reaction, thereby affecting a charge concentration in a semiconductor inversion layer. The change in the charge concentration can be detected by measuring a change in current. In the biosensor for detecting biological reactions of the prior art disclosed in U.S. Pat. No. 4,238,757, a protein is used as a biomolecule in the biosensor.
U.S. Pat. No. 4,777,019 discloses a biosensor based on a principle in which biological monomers are adsorbed on a surface of a gate and a degree of hybridization of the biological monomers with their complementary monomers may be measured using a FET to detect a presence of biological molecules.
FIG. 1 is a cross-sectional view of a FET of the prior art for detecting biomolecules. Referring to FIG. 1, a source 12 and a drain 13 are formed in both sides of a substrate 11 doped with an n- or p-type impurity, and each have polarities opposite to a polarity of the substrate 11. A channel 15 is formed between the source 12 and the drain 13, and an insulating layer 14 contacting the source 12 and the drain 13 is formed on the substrate 11. A reference electrode 16 is disposed above the insulating layer 14. A predetermined voltage is applied to the reference electrode 16.
A liquid sample (not shown) containing biomolecules comes into contact with the insulating layer 14 and the reference electrode 16. An amount of current flowing between the source 12 and the drain 13 changes according to a concentration of the biomolecules in the liquid sample, and a concentration of the biomolecules can therefore be obtained by measuring a change in the current flowing between the source 12 and the drain 13.
In the prior art, a concentration of biomolecules is detected using an array of FETs constructed as described above and by using the FET arrays in a plurality of chambers.
In the prior art, individual FETs of the plurality of FETs have different electrical characteristics, caused by a physical variation in device dimensions during a semiconductor manufacturing process, a variation in doping density during the semiconductor manufacturing process, and a variation in threshold voltage due to surface effects on the FETs, such as trapped charges in a gate oxide or surface states.
Further, the deviations of the electrical characteristics of the FETs increase as the FETs decreases in size or the distance between the FETs increases. The different electrical characteristics of the FETs adversely affect performance parameters such as precision, reproducibility and resolution of biomolecule detection.
FIG. 2 is a graph of voltage illustrating deviations of electrical characteristics of FETs constructed as the FET of the prior art in FIG. 1.
Referring to FIG. 2, an array of 45 conventional FETs is disposed in three chambers. More specifically, FETs 1-15 are disposed in Chamber 1, FETs 16-30 are disposed in Chamber 2 and FETs 31-45 are disposed in Chamber 3, as shown in FIG. 2. A voltage of 1.695 V was input to each of the 45 FETs. Although an output voltage of 1.695 V was expected, the output voltages of the conventional FETs were different from one another due to the electrical characteristic differences of the conventional FETs described above. An average output voltage was 1.699 V, a standard deviation was 5 mV, and a difference between a maximum output voltage and a minimum output voltage was 23.7 mV, as shown in FIG. 2. Thus, there is a significant possibility that an error in measurement results may occur due to the electrical characteristic differences of the FETs of the prior art.